The present invention is directed to a method and apparatus for estimating a wireless communication channel. More particularly, the present invention is directed to a method and apparatus for providing robust channel estimation in a multicarrier communication system.
As wireless communications demands continue to grow the goal is to be able to provide effective communication channels capable of transporting a wide range of information. As one example it has become desirable to provide wireless services capable of transporting multimedia information such as, for example high definition television. Multimedia wireless services require high-bit-rate transmission over radio channels. Those radio channels can provide mobile communications. One technique for reducing the effect of intersymbol interferences (ISI) caused by the dispersive Rayleigh fading environment is to make the symbol duration much larger than the channel delay spread. This can be achieved using a multicarrier configuration. One example of such a configuration is orthogonal frequency division multiplexing (OFDM) on which the entire channel is divided into many narrow subchannels, which are transmitted in parallel, thereby, increasing the symbol duration and reducing the ISI.
Orthogonality is a property of a set of functions such that the integral of the product of any two members of the set taken over the appropriate interval is zero. (For discrete functions summing is done rather than integrating.) For example, trigonometric functions appearing in Fourier expansions (e.g., sines and cosines) are orthogonal functions.
Orthogonality ensures that a receiver demodulating a selected carrier demodulates only that carrier without simultaneously and unintentionally demodulating the other carriers that are providing parallel data transmission along the multiplexed communication channel. Accordingly, there is no cross talk between carriers even though the carrier spectra overlap and there is no requirement of explicit filtering.
Therefore, OFDM is an effective technique for combating multipath fading and for providing high-bit-rate transmission over mobile wireless channels.
It is known to use differential demodulation, for example differential phase shift keying (DPSK) in OFDM systems to avoid system complexities that would otherwise arise from having to track a time varying channel. This differential demodulation technique comes at the expense of a 3-4 dB loss in signal-to-noise ratio (SNR) compared with coherent demodulation. If there is an accurate estimate of the channel characteristics that can be used to adjust the received signal, then the channel OFDM system performances can be improved by allowing for coherent demodulation. Furthermore, for systems with receiver diversity, optimum combining can be obtained by means of channel estimators.
It is well known that the structure of an OFDM signal allows a channel estimator to use both time correlation and frequency correlation. An ideal estimator would have a two dimensional structure that attempts to estimate the channel response in time and frequency.
It is suggested in one paper that such a two-dimensional estimator structure is generally too complex for a practical implementation. See OFDM CHANNEL ESTIMATION BY SINGULAR VALUE DECOMPOSITION, Edfors, et al. VTC '96, pp. 923-927. Additionally, it is known in the art to provide an estimator using only frequency correlation to avoid the complexities otherwise involved in utilizing time and frequency correlation. In one example an estimator considers minimum mean square error (MMSE) and least squares (LS) estimations.
It would be advantageous to provide estimation that takes advantage of both the time and frequency correlation of the channel while avoiding the complexities alluded to in the prior art. It would also be advantageous if that estimation could be robust, that is the estimator would provide good transmission characteristics over a widely varying channel which can easily arise in a mobile communication environment.